you can get on early and have dinner, maybe lunch...but you would have to tote your carry on till 1 or 2 oclock, which is usually when they have the rooms made up after the people disembark at 8am. I didnt realize that if you get there early you still are catered to.
half of the passengers when i went were spanish and most were on board when we checked in at 3:00.
getting off was a bigger problem ...for us there was a labor strike at the pier when we returned and no transportation or dock workers were allowed in....we got to stay on till 12:00 and have lunch till the union reached a deal...princess had scabs and thier employees unloading so we watched them leave first...didnt bother us , we had a hotel before and after our cruise.
San Juan is a rude place with suicidal cab drivers just getting to and from the airport/ pier is an adventure.